You just finished building your house on top of that epic mountain you found and just as you are about to go inside and sleep, you get ambushed by creepers. With little to no room to attack the invading creepers without them destroying your house in the process, you run to the edge of the cliff and jump...but what's this? You safely landed on some sponges at the bottom and took no fall damage? Great!

I know it's not the greatest idea but it would be nice to have a block that negates fall damage besides water.

I like your creativity, but two problems:
1) Sponges would be located in or near water, therefore eliminating the use of the sponge, because the water could negate it instead,
2) Sponges already have a use. They're a creative-only block used to clear water, which is often used in adventure maps to clear water to make building space.